$(\sin^2 \theta + \cos^2 \theta)(\sin\theta) = \; ?$
We can use the identity ${\sin^2 \theta} + {\cos^2 \theta} = 1$ to simplify this expression. $1$ ${\sin\theta}$ ${\cos\theta}$ $\theta$ We can see why this is true by using the Pythagorean Theorem. So, $(\sin^2 \theta + \cos^2 \theta)(\sin\theta) = 1 \cdot \sin\theta = \sin\theta$